This invention relates to a process for condensing trace amounts of xenon which are contained in liquid oxygen which has been vented from a main condenser of an air liquefying/separating unit, and for controlling the concentration of the xenon thus obtained at a given level. More particularly, the present invention relates to the condensation of xenon from liquid oxygen where the xenon concentration is controlled at a given level, even when the concentration of xenon in the liquid oxygen supplied from the main condenser of an air liquefying/separating unit fluctuates.
Xenon, a rare, gaseous element is used in numerous applications including in flash and fluorescent lamps, as a laser source, and in anesthetics. Xenon exists in ambient air at a level of 0.087 ppm. Highly purified xenon can be manufactured from liquified air via repeated separation processes.
Currently, xenon is recovered from liquid oxygen vented into the main condenser from the upper rectifying column in a large-scale air liquefying/separation unit. However the fraction of xenon found in the liquid oxygen vented from the main condenser is inversely proportional to the production amount of liquid oxygen.
Conventionally, xenon contained in liquid oxygen is recovered by a rectifying process. Recently a number of methods of so doing have been proposed. In one, reported in Fuji TechnoSystem Technical Report (61-2-1), 1986, pp. 430-431, rectification is used to condense xenon from liquid oxygen, using controlled concentrations of xenon with hydrocarbon groups removed through a catalytic combustion reaction in order to avoid the danger of explosion due to condensation of hydrocarbons (particularly methane) contained in the liquid oxygen.
In another proposed process, described in Tokko-Sho, 47-22937, oxygen and argon are exchanged in an argon exchanging column. This is followed by condensation of xenon via a rectifying process. Another process, described in Tokkai-Sho, 57-95583, involves condensation and rectification of xenon following exchange of high pressure nitrogen with oxygen. All these processes suffer from the common drawback of requiring multi-stage rectifying processes which, in turn, increases the costs of equipment and processing and therefore reduces the efficiency of xenon recovery.
The present inventors have filed a patent application, disclosed in Tokkai-Sho, 62-297206, in which xenon containing liquid oxygen, vented from the main condenser in the upper rectification column of an air separating unit, is subsequently fed into multiple adsorption columns filled with an adsorbent which selectively adsorbs the xenon. A series of adsorbing-desorbing steps gradually condenses the xenon, resulting, finally. in the recovery of highly purified xenon. This method overcomes many disadvantages and difficulties associated with conventional processes for xenon production.
However, due to the current trend of ever-growing demand for liquid oxygen, the production amounts of liquid oxygen is increasing. Many companies now set the lower limit guideline for liquid oxygen production at 1% of the total oxygen gas produced, and produce liquid oxygen at levels above 1%. Some companies increase the production amounts of liquid oxygen during the night shift, when the cost of electricity is lower. The increase in production amounts of liquid oxygen can result in problems when using the xenon production process described in Tokkai-Sho, 62-97206. These include (1) reduced xenon concentration in the increased production amounts of liquid oxygen (2) requirements for scaling up production equipment and (3) decreasing efficiency of xenon recovery due to variations in the amounts of liquid oxygen fed into the xenon recovery system.
Another process for xenon recovery, described in Tokko-Sho, 63,33634, involves condensation and rectification of xenon while exchanging it against oxygen gas containing lower hydrocarbons, following first stage xenon condensation and rectification. This process, like the one described in Tokkai-Sho, 62-297206, suffers from the current increase in production amounts of liquid oxygen by not being able to accommodate to wide fluctuations in the amounts of liquid oxygen fed into the condensing column.